The present invention relates to composite abrasive materials, and particularly composite abrasive materials of silicas and chemical cleaning agents that are useful in oral compositions, and which provide enhanced chemical cleaning action on teeth to add improved anti-calculus effect, as well as methods of producing and using such composite materials.
Modern dentifrices often contain an abrasive substance for controlled mechanical cleaning and polishing of teeth, and optionally a chemical cleaning agent, among other common ingredients, such as humectants, flavors, therapeutic ingredients, such as a fluoride source, rheology control agents, binders, preservatives, colors, and sudsing agents, among others.
The primary function of an abrasive substance in such dentifrice formulations is to help remove various deposits including pellicle film from the surface of teeth. Pellicle film is tightly adhered and often contains brown or yellow constituents, which impart an unsightly appearance to the teeth. However, while cleaning is important the abrasive typically is selected so as not to be overly abrasive as to damage hard tissues of teeth.
As the abrasive material, synthetically produced amorphous precipitated silicas have played an important role as an ingredient in many contemporary dentifrice formulations. In addition to their cleaning ability, they are also safe, nontoxic, and compatible with other standard dentifrice ingredients, such as glycerin, sorbitol (or xylitol), thickening agents, detergent coloring and fragrance materials and, optionally, fluoride and other therapeutically active compositions.
Synthetic amorphous precipitated silicas are generally prepared by admixing alkaline silicate solutions with acids, stirring and then filtering out the precipitated silica. The resulting precipitate is next washed, dried, and often comminuted to a desired size. When preparing synthetic precipitated silicas, the objective is to obtain silicas, which provide maximal cleaning with minimal damage to oral (hard) tissues of the teeth. Dental researchers are continually concerned with identifying precipitated silicas meeting these objectives. Examples of the many patented publications describing such precipitated silicas include U.S. Pat. Nos. 4,122,161, 5,279,815 and 5,676,932 to Wason et al., and U.S. Pat. Nos. 5,869,028 and 5,981,421 to McGill et al.
As prior chemical cleaning agents for teeth, alkali metal salts of tartaric acid, citric acid, and soluble pyrophosphates have been added neat (as a separate ingredient from the abrasive) during the actual formulating of the dentifrice composition. Chemical cleaning agents include water soluble salts that are thought to chelate calcium ions as a mechanism for inhibiting calculus formation on teeth. As explained in U.S. Pat. Nos. 4,806,340 and 4,515,772, dental calculus, or tartar as it often is referred, is a hard, mineralized deposit which forms on the surfaces of teeth, especially at the gingival margin. Mature calculus is formed on the teeth when crystals of calcium phosphates begin to be deposited in the pellicle and extracellular matrix of the dental plaque and become sufficiently closely packed together for the aggregates to become resistant to deformation. The resulting crystalline calculus material is arranged in a hydroxyapatite crystal lattice structure, but an organic portion is also present including epithelial cells, food debris, microorganisms, and so forth.
Chelating agents, such as certain pyrophosphates, have been added neat to dentifrices in efforts to chelate calcium ions found in the cell walls of the bacteria on teeth and also disrupt, inhibit and reduce plaque and calculus formation by removing calcium from the calcium bridges which help hold the calculus biomass intact. As with the abrasives, the chelating agents, should not be too aggressive. A chelating agent having too high an affinity for calcium may cause undesired tooth demineralization.
Thus, an effective dentifrice formulation should maximize pellicle film removal without causing undue abrasion or dimineralization of the hard teeth tissue. According to U.S. Pat. Nos. 5,869,028 and 6,238,648, and Pader, M., Oral Hygiene Products and Practice, Marcel Dekker, Inc., New York, 1988, pp. 365-368, an effective amount of pyrophosphate salt added neat to conventional dentifrices in efforts to impart such chemical cleaning effects has been considered to be enough to provide at least about 1.0% free pyrophosphate ions, and an amount in the range of about 1.5-2.0% to about 6% is often preferred. According to U.S. Pat. No. 6,238,648, the pyrophosphates can be added to dentifrices in their anhydrous or hydrated forms, and may be present predominantly dissolved, predominantly undissolved, or as a mixture of those physical states.
The cleaning properties of dentifrice compositions on teeth are typically expressed in terms of Pellicle Cleaning Ratio (PCR) described by Stookey, et al., J. Dent. Res., 61, 1236-1239, and Hefferren, J. J., J. Dent. Res., 37, 563-573. However, the traditional PCR test was developed at a time when chemical cleaning agents were not used in dentifrice formulations.
Generally speaking, the PCR test begins with a biological film applied to extracted bovine teeth and the tooth is kept moist until tested. The brightness (L value) of each tooth is measured after the stain is applied. The teeth are then brushed with a slurry of the sample toothpaste. After brushing, the brightness of each tooth is again measured and the difference in values (xcex94L) used to determine the cleaning effectiveness of the toothpaste.
The primary measure of cleaning in the PCR test is the result of mechanical cleaning. The abrasiveness of the toothpaste is measured. The abrasiveness of the toothpaste results from the silica, calcium phosphate, alumina, or other solid particles in the toothpaste mechanically removing the film on the bovine teeth used in the test. In general, the type of stain used and the duration of the brushing in the PCR does not allow for sufficient contact time for chemical cleaning to occur.
As noted previously, the conventional method for incorporating chemical cleaning agents in oral cleaning compositions, such as toothpaste, is to add them as separate ingredients during the formulating of the toothpaste. However, in spite of the many prior disclosures relating to compositions for oral cleaning and antiplaque activity in this respect, there is still a need for oral cleaning compositions providing combined and improved effects of pellicle cleaning and calculus inhibition.
Accordingly, it is an object of the present invention to provide chemical cleaning agents adapted for use in oral cleaning compositions in a manner which can impart improved effects of pellicle cleaning and calculus inhibition. Another object of the present invention is to provide an abrasive cleaning material useful for preventing or removing tooth stains, and thereby whitening teeth. A further object of the present invention is to provide an abrasive cleaning material useful for preventing or removing plaque from teeth. It is yet another object of the invention to provide an abrasive cleaning material having chemical cleaning functionality that is useful in oral cleaning compositions as well as being generally useful in other types if cleaning applications. These and other objects will become readily apparent from the disclosure which follows.
The present invention relates to a composition of matter comprising particles of silica characterized by the silica particles having surfaces at which condensed phosphate is retained. The silica surfaces at which the condensed phosphate is retained on the silica particles comprise land areas, surface pores, or both. For purposes herein, this composition of matter is often referred to as surface-modified silica particles of the invention.
Among other things, the surface-modified silica particles of this invention impart enhanced chemical cleaning, stain removal, anti-calculus, and/or antiplaque properties when incorporated into oral cleaning compositions, to yield a significant increase and improvement in teeth whitening.
In one aspect, the silica used as the substrate that is surface-modified by the condensed phosphate can be precipitated silica or silica gel. In another aspect, the condensed phosphate is one or a combination of tetraalkali metal pyrophosphate, dialkali metal diacid pyrophosphate, trialkali metal monoacid pyrophosphate, pentaalkali metal tripolyphosphate, and alkali metal polymetaphosphate. In one preferred aspect, the condensed phosphate is one or a combination of tetrasodium pyrophosphate, tetrapotassium pyrophosphate, disodium dihydrogen pyrophosphate, trisodium monohydrogen pyrophosphate, pentasodium tripolyphosphate and sodium polymetaphosphate, singly or in combinations thereof.
In a further aspect, the surface-modified silica of this invention comprises about 30 to about 99.9 wt % silica and from about 0.1 to about 70 wt % condensed phosphate deposited at the silica surfaces. In one further aspect, the surface-modified silica of this invention comprises about 60 to about 95 wt % silica and from about 5 to about 40 wt % condensed phosphate. In yet another aspect, the surface-modified silica particles of this invention have an average particle size value ranging from about 0.1 to about 80 xcexcm.
In another aspect, the present invention provides a method for producing such surface-modified silica particles in which the solid silica substrate particles to be surface-modified are produced by acidulation of an alkali metal silicate. The crude silica particles obtained are then contacted with the condensed phosphate salt as dissolved in an aqueous medium. The silica continues to be solid particles during this mixing. This water-containing mixture is dried effective to form silica-based particles having surface-modification due to the presence (association) of the condensed phosphate in the mixture. In a preferred embodiment, the silica particles are not dried to a water content of less than about 15 wt % water until after being bathed with the aqueous solution containing the dissolved cleaning agent. In one preferred implementation of this embodiment, the drying of the surface-modified silica is performed by spray drying.
The resulting dried, surface-modified silica particles of the method of this invention are freely-flowable in bulk form, and are stable in a dry or wet state. In the dried state, the particles can be stably stored and shipped as needed for later use when desired in formulating an oral cleaning composition, such as a toothpaste. Even after the modified silica particles are actually incorporated into a dentifrice composition, the resulting dentifrice formulations are observed to have sufficiently long shelf lives for practical usage. The dried cleaning agent deposits on the silica surfaces are retained until released during brushing. The association and retention thereof is substantially retained intact thereon even after numerous months of presence in a water-containing dentifrice.
Another advantage gained by the surface-modified silica particles made according to this invention is that lower concentrations of the cleaning agent, that is, the condensed phosphates, can be used to provide equivalent cleaning in a dentifrice as compared to that of dentifrices using higher concentrations of the same cleaning agent except as added in the conventional neat (free) form.
The oral cleaning compositions that can be benefited by incorporation of the surface-modified silica particles of this invention include, for example, liquid dentifrices, toothpastes, chewing gums and mouthwashes, and the like. The surface-modified silica particles of the invention also have wider cleaning utility and application, including, for instance, as a metal, ceramic or porcelain cleaning or scrubbing agent.